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Structural Foundations for Bridges
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AASHTO LRFD:AASHTO LRFD:Structural Foundations and Earth Structural Foundations and Earth
Retaining StructuresRetaining Structures Specification Background Specification Background What’s What’s
Happening Now!Happening Now!
Limit States, Soil and Rock PropertiesLimit States, Soil and Rock Properties Deep FoundationsDeep Foundations Shallow FoundationsShallow Foundations Earth Retaining StructuresEarth Retaining Structures
Jerry DiMaggio, P. E., Principal Bridge Engineer Jerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical)(Geotechnical)
Federal Highway AdministrationFederal Highway AdministrationOffice of Bridge TechnologyOffice of Bridge TechnologyWashington D.C.Washington D.C.
? New Legal Load
AASHTO Specification Background: AASHTO Specification Background: Geotechnical Engineering PresenceGeotechnical Engineering Presence
* TRB/ NCHRP Activities (A LOT!)* TRB/ NCHRP Activities (A LOT!)* * Geotechnical Engineering does NOT have a Geotechnical Engineering does NOT have a
broad based presence on AASHTO broad based presence on AASHTO SubCommittees and Task Forces as do other SubCommittees and Task Forces as do other technical specialties.technical specialties.
* SubCommittee on Construction (guide * SubCommittee on Construction (guide construction specs) construction specs)
* SubCommittee on Materials (specs on * SubCommittee on Materials (specs on
materials and testing standards) materials and testing standards)
* SubCommittee on Bridges and Structures * SubCommittee on Bridges and Structures (specs on materials/ systems, design, and (specs on materials/ systems, design, and construction) construction)
History of AASHTO: Design & History of AASHTO: Design & Construction Specifications for Construction Specifications for Bridges and StructuresBridges and Structures
* First structural * First structural “Guideline Specification”“Guideline Specification” early early 1930s1930s
(A code yet NOT A code!).(A code yet NOT A code!).* First “significant” Geotechnical content * First “significant” Geotechnical content 1989.1989.* First LRFD specification 1994 (Current – 2004, * First LRFD specification 1994 (Current – 2004,
33rdrd edition). edition).* * First REAL Geotechnical involvementFirst REAL Geotechnical involvement in Bridge in Bridge
SubCommittee activities @ SubCommittee activities @ 1996.1996. (Focus on (Focus on mse walls).mse walls).
* Technical advances to Standard Specifications * Technical advances to Standard Specifications STOPPED STOPPED in 1998in 1998 to encourage LRFD use to encourage LRFD use (secret).(secret).
* Major rewrites needed to walls and * Major rewrites needed to walls and foundations sections (NOW COMPLETE).foundations sections (NOW COMPLETE).
““Geotechnical Scope”:Geotechnical Scope”: AASHTO AASHTO Design & Construction Specifications Design & Construction Specifications for Bridges and Structuresfor Bridges and Structures
* * Topics Included:Topics Included: Subsurface Subsurface Investigations, soil and rock properties, Investigations, soil and rock properties, shallow foundations, driven piles, drilled shallow foundations, driven piles, drilled shafts, rigid and flexible culverts, shafts, rigid and flexible culverts, abutments, WALLS (cantilever, mse, crib, abutments, WALLS (cantilever, mse, crib, bin, anchor).bin, anchor).
* * Topics NOT addressedTopics NOT addressed: integral : integral abutments, micropiles, augercast piles, abutments, micropiles, augercast piles, soil nails, reinforced slopes, and ALL SOIL soil nails, reinforced slopes, and ALL SOIL and ROCK EARTHWORK FEATURES. and ROCK EARTHWORK FEATURES.
Standard and LRFD AASHTO Standard and LRFD AASHTO SpecificationsSpecifications* Currently AASHTO has 2 separate * Currently AASHTO has 2 separate
specifications: Standard specs 17specifications: Standard specs 17thth edition and LRFD, 2004 3rd edition.edition and LRFD, 2004 3rd edition.
* Standard Specifications use a * Standard Specifications use a combination of working stress and load combination of working stress and load factor design platform.factor design platform.
* LRFD uses a limit states design * LRFD uses a limit states design platform with platform with different load and resistance different load and resistance factorsfactors (than LFD). (than LFD).
LRFD IMPLEMENTATION STATUSLRFD IMPLEMENTATION STATUS
Geotechnically, most States still use a Geotechnically, most States still use a working stress approach for earthworks, working stress approach for earthworks, structural foundations, and earth structural foundations, and earth retaining structures. Several States have retaining structures. Several States have totally adopted LRFD.totally adopted LRFD.
Many State Geo/Structural personnel and Many State Geo/Structural personnel and consultants consultants ARE NOT FAMILARARE NOT FAMILAR with the with the content of LRFD 3content of LRFD 3rdrd edition. edition.
““AASHTO and FHWA have agreed that AASHTO and FHWA have agreed that all state DOTs will use LRFD for NEW all state DOTs will use LRFD for NEW structure design by 10/07.”structure design by 10/07.”
What are UNIQUE Geotechnical issues What are UNIQUE Geotechnical issues related to LRFD?related to LRFD?
* Strong influence of construction on design.* Strong influence of construction on design.
* GEOTECHs strong bias toward performance * GEOTECHs strong bias toward performance based specifications.based specifications.
* Natural variability of GEO materials.* Natural variability of GEO materials.
* Variability in the type, and frequency of tests, * Variability in the type, and frequency of tests, and method to determine design property and method to determine design property values of soil and rock.values of soil and rock.
* Differences between earthwork and structural * Differences between earthwork and structural foundation design model approaches.foundation design model approaches.
* Influence of regional and local factors.* Influence of regional and local factors.
* * General lack of data on limit state conditions.General lack of data on limit state conditions.
What Happening Now?
* FHWA sponsored a complete rewrite of Section 10 * FHWA sponsored a complete rewrite of Section 10 during 2004. The rewrite was prepared by National during 2004. The rewrite was prepared by National subject matter experts and had broad input from a subject matter experts and had broad input from a number of Key State Dots, (including T-15 member number of Key State Dots, (including T-15 member States), and the Geotechnical community (ASCE - States), and the Geotechnical community (ASCE - GI, DFI, ADSC, PDCA).GI, DFI, ADSC, PDCA).
* During the Proposed spec development @ 2000 * During the Proposed spec development @ 2000 comments were addressed. The Proposed spec comments were addressed. The Proposed spec was then distributed to all States for review. An was then distributed to all States for review. An additional @ 1000 comments were addressed.additional @ 1000 comments were addressed.
* The revised Proposed Specification was advanced * The revised Proposed Specification was advanced and approved by the AASHTO’s Bridge and and approved by the AASHTO’s Bridge and Structures Sub-Committeee in June 2005. Structures Sub-Committeee in June 2005. The revised Proposed Specification is used in the The revised Proposed Specification is used in the NHI LRFD Substructure course which currently NHI LRFD Substructure course which currently available.available.
Fundamentals of LRFDFundamentals of LRFDPrinciples of Limit State DesignsPrinciples of Limit State Designs
* Define the term “Limit State” * Define the term “Limit State”
* Define the term “Resistance”* Define the term “Resistance”
* Identify the applicability of each of the * Identify the applicability of each of the four primary limit states.four primary limit states.
* Understand the components of the * Understand the components of the fundamental LRFD equation.fundamental LRFD equation.
A Limit State is a A Limit State is a defined conditiondefined condition beyond which a structural component, beyond which a structural component,
ceases to satisfy the provisions for which ceases to satisfy the provisions for which it is designed.it is designed.
Resistance is a Resistance is a quantifiable value that quantifiable value that definesdefines the point beyond which the the point beyond which the
particular limit state under investigation particular limit state under investigation for a particular component will be for a particular component will be
exceeded.exceeded.
Resistance can be defined in Resistance can be defined in terms of:terms of:
* Load/Force (static/ dynamic, dead/ * Load/Force (static/ dynamic, dead/ live)live)
* Stress (normal, shear, torsional)* Stress (normal, shear, torsional)
* Number of cycles* Number of cycles
* Temperature* Temperature
* Strain* Strain
Limit StatesLimit States
* Strength Limit State* Strength Limit State
* Extreme Event Limit * Extreme Event Limit StateState
* Service Limit State * Service Limit State
* Fatigue Limit State* Fatigue Limit State
LIST
Strength Strength Limit Limit StateState
Extreme Event Limit Extreme Event Limit StateState
Service Limit StateService Limit State
Service Limit Service Limit StateState
Rn / FS Rn / FS QQ
iiiiQQii ≤ R≤ Rrr = = RRnn
ii ==
ii ==
QQii ==
RRrr = =
==
RRnn ==
Load modifier (eta)Load modifier (eta)
Load factor (gamma)Load factor (gamma)
Force effectForce effect
Factored resistanceFactored resistance
Resistance factor (phi)Resistance factor (phi)
Nominal resistanceNominal resistance
iiiiQQii ≤ R≤ Rrr = = RRnn
ff((,,))
QQnn RRnn
QQ RR
QQnn
RRnn
Q or RQ or R
Pro
babili
ty o
f Pro
babili
ty o
f O
ccurr
ence
Occ
urr
ence
Subsurface MaterialsSubsurface Materials
* Soil* Soil
* Rock* Rock
* Water* Water
* Organics* Organics
10.4 SOIL AND ROCK PROPERTIES10.4.1 Informational Needs10.4.2 Subsurface Exploration10.4.3 Laboratory Tests10.4.3.1 Soil Tests10.4.3.2 Rock Tests10.4.4 In-situ Tests10.4.5 Geophysical Tests10.4.6 Selection of Design Properties10.4.6.1 Soil Strength10.4.6.1.1 Undrained strength of Cohesive
Soils10.4.6.1.2 Drained Strength of Cohesive Soils10.4.6.1.3 Drained strength of Granular Soils10.4.6.2 Soil Deformation10.4.6.3 Rock Mass Strength10.4.6.4 Rock Mass Deformation10.4.6.5 erodibility of rock
Soil CharacteristicsSoil Characteristics
* Composed of individual grains of * Composed of individual grains of rockrock
* Relatively low strength* Relatively low strength
* Coarse grained (+ #200)* Coarse grained (+ #200)* High permeability* High permeability
* Fine grained (- #200)* Fine grained (- #200)* Low permeability* Low permeability
* Time dependant effects* Time dependant effects
Rock CharacteristicsRock Characteristics
* Strength* Strength* Intermediate * Intermediate
geomaterials,geomaterials,qquu = 50-1500 = 50-1500 psipsi
* Hard rock, * Hard rock, qquu > 1500 psi > 1500 psi
* Rock mass * Rock mass propertiesproperties
Undrained Strength of Undrained Strength of Cohesive Soils, sCohesive Soils, suu
Unconfined CompressionUnconfined Compressionssuu = q = quu/2/2
Vane Shear TestVane Shear Test
ssuu
qquu
=0=0
Typical ValuesTypical Valuesssuu = 250 - 4000 psf = 250 - 4000 psf
Drained Strength of Drained Strength of Cohesive Soils, c’ and Cohesive Soils, c’ and ’’ff
Triaxial Triaxial Compression Compression
CU TestCU Test
Typical Valuesc’ = 100 - 500 psf
’f = 20o - 35o
For NFor N116060 = 10, select = 10, select ’’ff = 30 = 30oo
(modified after Bowles, 1977)(modified after Bowles, 1977)
NN116060 ff
<4<4 25-3025-30
44 27-3227-32
1010 30-3530-35
3030 35-4035-40
5050 38-4338-43
Soil DeformationSoil Deformation
00
-2-2
-4-4
-6-6
-8-8
-10-10
-12-1211 1010 100100 10001000 1000010000
Time (days)Time (days)
Sett
lem
en
t (i
n)
Sett
lem
en
t (i
n) Initial elastic settlement (all soils)Initial elastic settlement (all soils)
Primary consolidationPrimary consolidation Secondary consolidationSecondary consolidation
Fine-grained (cohesive) soilsFine-grained (cohesive) soils
Consolidation PropertiesConsolidation Properties
LogLog1010 vv’’
Void
Rati
o (
e)
Void
Rati
o (
e)
pp’ = ’ = Preconsolidation Preconsolidation StressStress
CCss
CCrr
CCcc
0.10.1 11 1010 100100
0.50.5
11eeoo
One log cycleOne log cyclee=Ce=C=0.06=0.06
0.10.1 11 1010 100100 10001000 1000010000
Elapsed Time (min)Elapsed Time (min)
Void
rati
o (
e)
Void
rati
o (
e)
2.652.65
2.62.6
2.552.55
2.52.5
2.452.45
2.42.4
2.352.35
2.32.3
2.252.25
Stress Range, 40 – 80 kPaStress Range, 40 – 80 kPa
ttpp
Elastic Properties of SoilElastic Properties of Soil
Young’s Modulus, EYoung’s Modulus, Ess Typical values, 20 – 2000 tsfTypical values, 20 – 2000 tsfPoisson’s Ratio, Poisson’s Ratio, Typical values, 0.2 – 0.5Typical values, 0.2 – 0.5Shear Modulus, GShear Modulus, G Typical values, ETypical values, Ess / [2 (1 + / [2 (1 + )])]Determination by correlation to Determination by correlation to N1N16060 or s or suu, or in-situ tests, or in-situ tests
Rock PropertiesRock PropertiesLaboratory testing is for small intact Laboratory testing is for small intact rock specimens rock specimens Rock mass is too large to be tested Rock mass is too large to be tested in lab or fieldin lab or fieldRock mass properties are obtained Rock mass properties are obtained by correlating intact rock to large-by correlating intact rock to large-scale rock mass behavior – failures scale rock mass behavior – failures in tunnels and mine slopesin tunnels and mine slopesRequires geologic expertiseRequires geologic expertise
Intact Rock StrengthIntact Rock Strength
Point Load TestPoint Load Test
Unconfined Compression, qUnconfined Compression, quu
Typical ValuesTypical Valuesqquu = 1500 - 50000 psi = 1500 - 50000 psi
Rock QualityRock QualityLen
gth
, L
Len
gth
, L
0.8 ft0.8 ft
0.7 ft0.7 ft
0.8 ft0.8 ft
0.6 ft0.6 ft
0.2 ft0.2 ft
0.7 ft0.7 ft
SoundSound
Not soundNot sound, highly weathered, highly weathered
Not soundNot sound, centerline pieces < 4 , centerline pieces < 4 inches, highly weatheredinches, highly weathered
SoundSound
Not soundNot sound
SoundSound
Core RunCore RunTotal = 4 Total = 4 ftft
CRCR = 95% = 95% RQD RQD = 53%= 53%
CSIR Rock Mass Rating CSIR Rock Mass Rating SystemSystem
This system is based on qThis system is based on quu, RQD, , RQD, joint spacing, joint condition and joint spacing, joint condition and water condition.water condition.
Rock Mass StrengthRock Mass Strength
CC11’’
Sh
ear
str
ess,
Sh
ear
str
ess,
Effective Normal Stress, Effective Normal Stress, ’’
tmtm 33 11
’’ii
= (cot = (cot ’’ii – cos – cos ’’ii)mq)mquu/8/8
’’ii = tan = tan-1-1(4 h cos(4 h cos22[30+0.33sin[30+0.33sin-1-1(h(h-3/2-3/2)]-1))]-1)-1/2-1/2
h = 1 + 16(mh = 1 + 16(m’’nn+sq+squu)/(3m)/(3m22qquu))
Intact Rock Deformation, EIntact Rock Deformation, Eii
Typical values range from 1000 to Typical values range from 1000 to 13000 ksi13000 ksi
Poisson’s Ratio, Poisson’s Ratio, Typical values range from 0.1 to 0.3Typical values range from 0.1 to 0.3
Rock Mass DeformationRock Mass Deformation
EE = 2 RMR - 100 = 2 RMR - 100
40
10RMR
M 10145,000E
9090
7070
5050
3030
1010
In s
itu
In s
itu m
od
ulu
s of
defo
rmati
on
, E
mod
ulu
s of
defo
rmati
on
, E
MM (
GPa)
(G
Pa)
1010 3030 5050 7070 9090
1212
1010
88
66
44
22
(psi x 10(psi x 1066))
Rock mass rating Rock mass rating RMRRMR
GEC 5GEC 5FHWA-IF-02-034FHWA-IF-02-034
Jerry A. DiMaggio P. E.Jerry A. DiMaggio P. E.Principal Bridge Engineer Principal Bridge Engineer TEL: (202) 366-1569TEL: (202) 366-1569FAX: (202) 366-3077FAX: (202) 366-3077
The best Geotechnical web The best Geotechnical web site in town! site in town!
www.fhwa.dot.gov/bridgewww.fhwa.dot.gov/bridge
WOW! FREE STUFF FROM WOW! FREE STUFF FROM THE FEDERAL THE FEDERAL
GOVERNMENT!GOVERNMENT!
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